An optical waveguide element using a substrate with an electro-optical effect of lithium niobate (hereinafter, abbreviated to LN) or the like has features in which a loss is smaller, an operation can be performed at a higher speed, and characteristics are more stable in a wide wavelength range than a semiconductor modulator. Therefore, the optical waveguide element is widely used, particularly, in a high speed optical communication system such as a wavelength multiplex optical transmission system.
In order to use the optical waveguide element in an actual system, a structure is necessary in which shift (temperature drift) of an operation point which changes due to a temperature variation or shift (DC drift) of an operation point due to long term DC voltage application is compensated for by using a feedback circuit. For this reason, several countermeasures for reducing an amount of the shift as much as possible have been implemented.
For example, as shown in PTL 1, a technique, in which influence of stress of an electrode or the like on the temperature drift is reduced through annealing, is disclosed. In addition, as shown in PTL 2, a technique, in which a buffer layer is doped with an impurity such as In so as to reduce the DC drift, is disclosed. However, the operation point shift of the LN modulator is influenced by an extremely delicate balance of stress, or a very small amount of impurity included in a crystal or a buffer layer and a balance thereof. Therefore, although various countermeasures including the above-described techniques have been proposed, it has not been realized yet to completely suppress the temperature drift and the DC drift of the optical waveguide element.
On the other hand, an optical waveguide element such as an LN optical modulator is required to be operated at a higher speed and to be driven at a lower voltage due to an increase in a desired transmission capacity. In addition, the optical waveguide element is required not only to perform relative simple intensity modulation such as an NRZ format but also to support a format in which a phase or polarization information can be transmitted simultaneously such as a DQPSK or a polarization multiplexing modulator. For this reason, a substrate configuration of the optical waveguide element becomes complicated such as a ridge structure or a thin plate structure, and a waveguide configuration also becomes complicated from a single Mach-Zehnder type to a nest type.
In accordance therewith, compensation of the above-described operation point shift using a feedback circuit also becomes complicated. Particularly, an optical waveguide element whose DC drift is reduced is desired to be able to control and compensate with lower voltage for the DC drift which is the essence of ensuring a long term operation.
Furthermore, on the other hand, in producing and supplying an optical waveguide element whose DC drift is suppressed to a low level, it is necessary to confirm corresponding characteristics would be satisfied with a system requirement specification by evaluating the DC drift and the like on a wafer in which elements are completed or by chips cut out of the wafer and selecting at a final inspection of a complicated wafer process. Typically, in a case where a wafer or a chip is defective in this stage, the wafer or the chip is discarded, and thus a cost loss due to a decrease in production yield occurs. This is because, if once the wafer process is completed, a wafer or a chip is just used or discarded by good or bad judgment for the finished product, and there is no technique for correcting or adjusting DC drift characteristics with additional adjustment after a wafer is completed, for example, in consideration of an evaluation result of a chip.
A mechanism of the DC drift is described by an equivalent circuit inside an optical waveguide element such as an LN modulator, for example, as in NPL 1. The important matter in this way is that a combined resistance of all of partial resistance values and a combined capacitance of all of partial electric capacitances and ratios of a resistance and a capacitance in each part in the direction of cross-section or surface of an optical waveguide, a buffer layer, a semiconductive film (a Si film or the like), and electrodes which are formed on an LN substrate have influence on long term shift of the DC drift. Therefore, in order to further reduce the DC drift, it is necessary to manufacture by accurately controlling resistances or capacitances in the direction of cross-section-and the surface of the LN substrate, the buffer layer, the semiconductive film, and the like, which are determined in each process, in addition to design of an electrode or a waveguide.
However, the LN material which is ferroelectric has lower crystallinity than a Si wafer or the like of a semiconductor and has a problem in which a variation is large depending on a manufacturer, a manufacturing lot, a manufacturing device, and the like. Variations of resistances in the direction of the cross-sectional or the surface are also large. Further, the resistance values thereof fluctuates to an extent in which a digit considerably changes just only by including a very small amount of impurity in a buffer layer or a semiconductor film which is formed in a wafer process. For this reason, it is very difficult to manufacture with accurately controlling resistances or capacitances in the direction of cross-section and the surface of the LN substrate, the buffer layer, the Si film, and the like, which are determined in each process during manufacturing of the LN modulator. Therefore, the DC drift of the optical waveguide element has some variation.
In addition, as a factor to make it more difficult to suppress reduction or variation of the DC drift, it is very difficult or substantially impossible to measure separately each resistances or capacitances or divide into each resistances or capacitances in the direction of the cross-section and the surface of the LN substrate, the buffer layer, the Si film, and the like, which are determined in each process. Accordingly, there is no realistic means except that the resistances or the capacitances are analogized from a combined resistance value and a combined capacitance of elements on a wafer during the process or of cut-out chips, a tendency or a degree of DC drift, and the like.
Therefore, the once finished wafer is only selected whether to transfer to a product assembly process or to discard by determining good or bad on the basis of characteristic evaluation of elements on a wafer or a cut-out chip.
FIG. 13 is a cross-sectional view illustrating a part of an optical waveguide element, in which an optical waveguide 12 is formed by forming a thermal diffusion portion of Ti and the like in a substrate 11 having an electro-optic effect. A signal electrode 13 and a ground electrode 14 are disposed near the optical waveguide 12 as a modulation electrode for applying an electric field to the optical waveguide.
In order to reduce a DC drift phenomenon, in PTL 3, it is proposed to stabilize characteristics by inserting the film for suppressing diffusion of Li between the substrate and the buffer layer as it is considered that Li from a substrate becomes a movable ion in a buffer layer as a factor of occurring the DC drift phenomenon.
In addition, in PTL 4, a contamination source which is entered from outside of an optical waveguide element is considered as a factor of the DC drift, and a method of forming a protective film on a buffer layer has been proposed in order to prevent a contamination source from entering the buffer layer.
Further, in PTL 5, stabilizing a DC drift characteristic is carried out by controlling an OH amount in a substrate or a buffer layer by doing annealing treatment in a dried gas atmosphere of oxygen.
However, in the technique related to PTL 3 or 4, a diffusion suppression layer or a protective layer is used to prevent penetration of an impurity which may become a movable ion, from the substrate or the outside mainly. These films are required to use the material in which a diffusion coefficient is small and it is hard to occur ion polarization, in order to show an effect thereof. For this reason, metal or semiconductor is mainly used as the material. If these kinds of materials are laid between the substrate in which an optical waveguide is formed and the buffer layer thereon, it becomes the cause to deteriorate characteristics like an optical loss or extinction ratio in the optical waveguide element due to refractive index or a light absorption effect of those materials.
In addition, similarly, also in a case where the material is laid on the buffer layer, there is a possibility that a propagation loss of a signal in the signal electrode or the like-or deterioration of applying efficiency of an electric field may cause. Therefore, this causes characteristics of the optical waveguide element to deteriorate. Further, a countermeasure like partially installed or so may be taken to prevent deterioration of the characteristics, but, in this case, pattern forming and the like using a technique of lithography and the like is required. Therefore, it makes a problem which productivity is poor.
In addition, there is a method of implanting an impurity into a buffer layer in order to reduce a resistance value of the buffer layer or to control the film quality. However, in this technique, the balance with a substrate or an electrode is problematic, it is difficult to stabilize characteristics because it is required to do its control strictly in case doing characteristic stabilization as an actual device.